A PUMP AND A SET OF SEALS SEALING THE STATOR COMPONENTS OF SUCH A PUMP

Abstract

A set of seals for sealing between two half shells defining at least one pumping chamber and two head plates to be mounted at either end of the two half shells. The set of seals includes: at least one annular seal for sealing between at least one of the head plates and the two half shells; two longitudinal seals for sealing between longitudinal contact faces of the two half shell stators on either side of the at least one pumping chamber, the longitudinal seals being configured to have end portions that abut against the at least one annular seal when mounted in the pump. Each of the seals within the set of seals are made of a material having at least one of: a hardness between 73-83, on the Shore A hardness scale; and a stiffness of between 2.4 and 4.2 N/mm per mm length.

Claims

1. A set of seals for sealing between the stator components of a pump, said stator components comprising two half shells defining at least one pumping chamber and two head plates to be mounted at either end of said two half shells, said set of seals comprising: at least one annular seal for sealing between at least one of said head plates and said two half shells; two longitudinal seals for sealing between longitudinal contact faces of said two half shell stators on either side of said at least one pumping chamber, said longitudinal seals being configured to have end portions that abut against said at least one annular seal when mounted in said pump; wherein each of said seals within said set of seals are made of a material having at least one of: a hardness between 73-83, preferably 75-81 on the Shore A hardness scale; and a stiffness of between 2.4 and 4.2 N/mm per mm length, wherein said stiffness is measured by axially compressing at room temperature an O-ring of said material, said O-ring having a cross section diameter of 3.4 mm and an internal diameter of at least 40 mm.

2. The set of seals according to claim 1, wherein said seals of said set of seals is made of a material having both of: a hardness between 74-80 on the Shore A hardness scale; and a stiffness of between 2.4 and 4.2 N/mm per mm length, wherein said stiffness is measured by compressing an O-ring of said material, said O-ring having a cross section diameter of 3.4 mm and a diameter of at least 40 mm.

3. The set of seals according to claim 1, wherein said stiffness of said material is between 2.6 and 3.8 N/mm per mm length.

4. The set of seals according to claim 1, wherein said stiffness of said material is between 2.8 and 3.4 N/mm per mm length.

5. The set of seals according to claim 1, wherein said material is a nitride or a fluorinated elastomer.

6. The set of seals according to claim 1, wherein said material is a material resistant to temperatures in excess of 200° C.

7. The set of seals according to claim 5, wherein said material comprises a perfluoroelastomer

8. The set of seals according to claim 1, wherein said annular seal and said longitudinal seals are formed from a same material.

9. The set of seals according to claim 1, wherein said longitudinal seal has a thickness of more than 1.5 mm.

10. The set of seals according to claim 1, wherein said longitudinal seal has a thickness of more than 1.8 mm, preferably more than 2.2 mm.

11. The set of seals according to claim 1, wherein at least one end of said longitudinal seal configured to abut against said annular seal comprises a flat end surface prior to said seal being compressed.

12. The set of seals according to claim 1, wherein said materials comprises: FFKM: Trelleborg Isolast© J9577

13. A pump assembly comprising a rotor rotatably mounted within a stator, said stator comprising: two half shells defining at least one pumping chamber; and two head plates mounted at either end of said two half shells; and a set of seals according to claim; wherein said longitudinal seals are mounted to seal between longitudinal contact faces of said two half shells and said at least one annular seal is mounted to seal between one of said head plates and an end of said two half shells, said longitudinal seals having end portions that abut against said at least one annular seal.

14. The pump assembly according to claim 13, wherein said pump assembly comprises a vacuum pump.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0046] FIG. 1 shows the measured stiffness of a plurality of test materials;

[0047] FIG. 2 shows the apparatus used to test the materials;

[0048] FIG. 3 schematically shows the stator components to be sealed by a set of seals according to an embodiment; and

[0049] FIG. 4 schematically shows the seals within the stator components.

DETAILED DESCRIPTION

[0050] Before discussing the embodiments in any more detail, first an overview will be provided.

[0051] Vacuum pumps with a clam-shell construction have T-joints between the stators (clams) and the headplates. These T-joints are sealed with a combination of axial and annular seals, which are compressed. The complex geometry of the T-seal causes the seals to distort when compressed. The main challenge is to keep the seals in contact with the housings and in contact with each other at all possible compressions and all operational temperatures.

[0052] Embodiments provide a specific T-seal design which reduces variation in the compression of the axial seals. Many elastomeric materials were evaluated in this T-seal design and the majority failed, a seal was deemed to fail where there is a leak rate above 1×10.sup.−6 mbar.Math.l/s. The failures have either been at room temperature or elevated temperatures such as 180° C. or 220° C.

[0053] It was observed that a predictor of failure was the elastomer stiffness and/or hardness of the material and that when it was in a specific range, the T-seals perform well across the required range of operating temperatures. Embodiments provide T-seals formed of a material with this specific range of stiffness and/or hardness.

[0054] In this regard, an elastomer stiffness that is too low causes the seals to distort excessively when compressed, which leads to an unpredictable interface between the axial and annular seals. An elastomer stiffness that is too high generates large variations in the sealing pressures, causing a T-seal leak when the compressions of the axial and annular seals are not balanced. Using a specific stiffness range ensures that a T-seal functions well over the full design compression ranges for the axial and annular seals.

[0055] It was also found that elastomer hardness properties provided a similar measure of suitability for the seals. However, stiffness provided a better predictor and therefore, a simple O-ring stiffness measurement was devised to identify suitable materials. This test involved the compression of the whole area of the O-ring under several loads.

[0056] The measured O-ring cross-section diameter was 3.4 mm+/−10%

[0057] The preferred stiffness is 3.12 N/mm per mm of O-ring length, however, materials in the range of 2.4 and 4.2 N/mm per mm length were found to have suitable properties.

[0058] A specific material that has this property and have worked well in T-seals is: [0059] FFKM: Trelleborg Isolast® J9577.

[0060] FIG. 1 shows a graph indicating force versus deflection for different materials tested using the O-ring compression test described previously.

[0061] In this regard the centreline length of each O-ring tested was between 135 and 145 mm and the determined stiffness for each of ref1 to ref5 was respectively: [0062] 3.12, 2.99, 2.00, 2.18, 5.60.

[0063] Thus, the materials ref1 and ref2 had the desired stiffness properties, that is a stiffness within the range 2.4 and 4.2 N/mm per mm length. The hardness of these two materials was 75 and 80 on the shore A scale and thus, this too was within the desired range and both values were a predictor of suitability.

[0064] FIG. 2 schematically shows the test apparatus used for determining the compression stiffness of the material. A test O-ring 12 having an internal diameter of more than 40 mm and a cross section diameter of 3.4+/−10% mm was tested for its stiffness by laying it on a flat surface and then placing another flat surface above the O-ring and compressing the O-ring with different loads 10 and measuring the deflection 15. The results were then plotted on a graph such as that shown in FIG. 1 and the stiffness determined therefrom.

[0065] FIG. 3 schematically shows a multiple chamber rotary pump assembly, with two stator half shells and end pieces, which assembly may be advantageously sealed using seals according to embodiments. The pump assembly is formed of two stator half shells 104 and 102 between which a rotor (not shown) is mounted. The two shells are fixed together to form the pump chambers. Each of the chambers 106, 108, 110, 112, 114 & 116 are separated by pump chamber walls 134. End pieces 122 and 124 are mounted on the half shells of the stators to complete the pump assembly. The longitudinal seals are mounted on the stator shell contact surfaces 118, 120, while the annular seals (not shown) are mounted on the mating surfaces of the end pieces 130, 132.

[0066] FIG. 4 shows an isometric view of the seals that are arranged between the stator half shells and between the end faces. In this embodiment, there are longitudinal seals 20 and 22 within grooves 50 on either side of the pumping chamber. There are also O-ring seals 40 and 42 also mounted within grooves between the end pieces and the stator half shells.

[0067] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.

[0068] Although elements have been shown or described as separate embodiments above, portions of each embodiment may be combined with all or part of other embodiments described above.

[0069] Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are described as example forms of implementing the claims.